This invention relates to a method for preparing catalysts useful in the manufacture of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly, it is directed to a method for the preparation of catalysts suitable for producing maleic anhydride from saturated hydrocarbons.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
The prior art teaches that vanadium catalysts are well suited to the production of maleic anhydride from hydrocarbons, and the prior art further teaches that phosphorus-vanadium-oxygen catalysts can be prepared in a number of ways. For example, these catalysts can be prepared by precipitating the vanadium and phosphorus compounds either with or without a carrier from a colloidal dispersion of the ingredients in an inert liquid, and thereafter calcining the precipitate. Catalysts can also be prepared by dissolving vanadium and phosphorus compound in a common solvent and thereafter depositing the resulting phosphorus-vanadium-oxygen compound from solution on a carrier.
Many prior art procedures for the preparation of vanadium catalysts teach that it is preferable to reduce the vanadium in solution to the tetravalent state. Hence, the prior art teaches that vanadium compounds can be contacted with a reducing acid, such as hydrochloric acid or oxalic acid, and then heated until the vanadium is reduced to a valence state of less than five before the compounds are subsequently recovered and used as catalysts.
Although the prior art procedures provide acceptable catalysts, there are attendant processing difficulties with such procedures. It has been discovered that phosphorus-vanadium-oxygen catalyst precursors can be prepared rapidly by reacting a trivalent phosphorus compound with a vanadium compound to provide tetravalent vanadium using elevated temperatures and pressures, such as are found in an autoclave. Although such procedures usually provide a finely divided solid catalyst precursor, from time to time the procedure forms large crystals of the precursor; upon forming a catalyst from such a precursor, low yields of maleic anhydride are obtained. These and other disadvantages are overcome by the present method for preparing phosphorus-vanadium-oxygen catalysts to consistently provide finely divided precursors that are converted to catalysts to provide high yields of maleic anhydride.